nfsd: remove nfsd_stats, make th_cnt a global counter

This is the last global stat, take it out of the nfsd_stats struct and
make it a global part of nfsd, report it the same as always.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>